Within the field of PSAs, continuing technological developments in the coating technique mean that there is an ongoing need for new developments. In industry, hotmelt processes with solventless coating technology are of increasing importance in the preparation of PSAs, since the environmental regulations are becoming ever more stringent and the prices of solvents continue to rise. Consequently solvents are to be eliminated as far as possible from the manufacturing operation for PSA tapes. The associated introduction of the hotmelt technology is imposing ever greater requirements on the adhesives. Acrylic PSAs in particular are the subject of very intensive investigations aimed at improvements. For high-level industrial applications polyacrylates are frequently preferred over other adhesive systems on account of their transparency and weathering stability. In addition to these advantages, however, these acrylic PSAs must also meet stringent requirements in respect of shear strength and bond strength. This profile of requirements is matched by polyacrylates of high molecular weight and high polarity with subsequent efficient crosslinking. These high shear strength, polar PSAs, however, possess the disadvantage that they are not well suited to the hotmelt extrusion operation: shearing within the extruder may degrade the polymer, with a consequent reduction in molecular weight. This damage significantly reduces the level of the adhesive properties. The shear strengths of the hotmelt-coated acrylic PSAs in particular fall distinctly in comparison to the original, solvent-coated PSA. At the present time, therefore, different concepts aimed at reducing the flow viscosity and thereby facilitating extrusion coating of these PSAs are being investigated.
The industry is pursuing a variety of concepts for achieving this objective. One possibility is the highly efficient crosslinking of a low viscosity, apolar acrylic adhesive not until it is on the backing. Acrylates containing electron-donating groups are copolymerized and, during crosslinking by UV or EBC (EBC: electron beam curing), they stabilize free radicals that are formed. Examples thereof are tertiary amine monomers [WO 96/35725], tertiary butylacrylamide monomer [U.S. Pat. No. 5,194,455], and tetrahydrofuryl acrylates [EP 0 343 467 B1]. A further efficient crosslinking concept is the copolymerization of UV photoinitiators into the polyacrylate chain. For example, benzoin acrylate has been used as a comonomer and the crosslinking has been conducted on the backing using UV light [DE 27 43 979 A1]. In U.S. Pat. No. 5,073,611, on the other hand, benzophenone and acetophenone were used as copolymerizable monomers.
Very efficient chemical crosslinking takes place by radiation in the case of polyacrylates containing double bonds [U.S. Pat. No. 5,741,543].
Styrene-isoprene-styrene (SIS) block copolymers, in contrast, are widespread elastomers for hotmelt processible PSAs [preparation processes: U.S. Pat. Nos. 3,468,972; 3,595,941; application in PSAs: U.S. Pat. Nos. 3,239,478; 3,935,338]. Good processing properties are achieved by virtue of a relatively low molecular weight and by virtue of a specific morphology [EP 0 451 920 B1]. These PSAs can be crosslinked very effectively with UV light in the presence of photoinitiators or with electron beams, since the middle blocks contain a multiplicity of double bonds.
Nevertheless these elastomers possess disadvantages, such as, for example, severe aging under UV light (in other words in daylight as well) and in an atmosphere containing oxygen/ozone. Another property which is very adverse for application is the relatively low thermal shear strength, owing to the use of polystyrene as hard block domain. These PSAs are therefore not suitable for relatively long-term outdoor bonds or for applications in relatively higher temperature ranges. The same is also true of other block copolymers which possess a middle block containing at least one double bond [U.S. Pat. No. 5,851,664].
One solution to the problem of aging, hotmelt processibility, high cohesion, and efficient chemical crosslinking by radiation is provided by the combination of SIS polymers with polyacrylates. Accordingly, US H1,251 describes diene copolymers containing acrylate for hotmelt applications, although these copolymers are likewise subject to aging, owing to the large number of double bonds which remain.
U.S. Pat. No. 5,314,962 describes A-B-A block copolymers as elastomers for adhesives, but these possess only A-domain formation as a cohesion-forming criterion and therefore lack great shear strength, especially at high temperatures.
EP 0 921 170 A1 describes A-B-A block copolymers which are modified with at least 40% resin additions. As a result of the high level of resin addition such PSAs are extremely hard and have virtually no tack. Moreover, the bond strengths as well, particularly on surfaces with a low energy, are lowered sharply.
It is an object of the invention, therefore, to provide improved pressure-sensitive adhesives based on polyacrylate which exhibit the disadvantages of the state of the art only to a reduced extent, if at all, and which therefore achieve adhesive properties, particularly bond strengths, which are improved as compared with conventional pressure-sensitive adhesives based on A-B-A block copolymers (A=hard block; B=polyacrylate and hence elastomer block) without losing the properties which are advantageous for their use as pressure-sensitive adhesives.